2008-10-08

Cellular Automaton and Agents in Second Life: Game of Life, Segregation and Evacuation Simulation

The Game of Life is a cellular automaton devised by the British mathematician John Horton Conway in 1970. It is perhaps the best-known example of a cellular automaton and as such formed the perfect starting point for looking at running models in Second Life.

The Game of Life sits alongside Schelling's Segregation Model and a Pedestrian Evacuation Model developed as part of a series of agent based models for our paper entitled 'Agent Street: Agent Based Modelling in Second Life'. The paper is currently about to be submitted for peer review/publication and should be available, subject to acceptance, late 2008.

The movie below details our Game of Life model running in Second Life:

The universe of the Game of Life is an infinite two-dimensional orthogonal grid of square cells, each of which is in one of two possible states, live or dead. Every cell interacts with its eight neighbours, which are the cells that are directly horizontally, vertically, or diagonally adjacent. At each step in time, the following transitions occur:

1. Any live cell with fewer than two live neighbours dies, as if by loneliness. 2. Any live cell with more than three live neighbours dies, as if by overcrowding. 3. Any live cell with two or three live neighbours lives, unchanged, to the next generation. 4. Any dead cell with exactly three live neighbours comes to life.

The initial pattern constitutes the 'seed' of the system. The first generation is created by applying the above rules simultaneously to every cell in the seed — births and deaths happen simultaneously, and the discrete moment at which this happens is sometimes called a tick. (In other words, each generation is a pure function of the one before.) The rules continue to be applied repeatedly to create further generations (ref Wikipedia)

Moving up a level of sophistication is Schelling's Segregation Model. Schelling originally demonstrated the concept with coins on a chess board. Using Second Life we have transferred the ideas utilising coloured spheres inside a grid of houses, as the movie below illustrates:

Schelling demonstrated how through mild tastes and preferences to locate in areas with similar people, large scale patterns of segregation could arise.

Finally we have developed in Second Life a pedestrian evacuation model. This is a much more accurate simulation of real life than the previous two models (the Game of Life and Segregation Model) and the rules that govern the agents are notably more complicated.

The clip above shows how agents exit a building once an alarm is sounded. Within the model, a model second is not the same as a real second so we have edited the clip so every model second is one real second just to give a sense of dynamics within the model.

Take a look at gisagents.blogspot.com for more thoughts on agent based modelling within both 2D and 3D environments, written by Andrew Crooks of CASA. Crooks is lead author of the forthcoming paper, his blog is the first place to look for a more in depth analysis of agent based modelling.

These are perhaps early steps but the ability to integrate human controlled avatars into models of building/environment evacuation, via environments such as Second Life, presents an intriguing step forward. Indeed, one that has the potential to lead to a more comprehensive understanding of evacuation behavior and ultimately of course better architectural design.